Silicones are synthetic poly(organo)siloxanes in which silicon atoms are linked via oxygen atoms (inorganic polymers). Silicones have already been used industrially for some decades. Only in recent years, however, have silicones gained increasing importance for optical uses. Primarily in automobile manufacture, in the field of LED lighting technology and for optics for solar applications, highly transparent materials which maintain their properties for a long time even at elevated temperature (>100° C.) are required. Silicones differ fundamentally from the class of thermoplastics and also from the class of inorganic glasses. As inorganic polymers, they form solid phases far above the glass transition point, while both thermoplastics and inorganic glasses are geometrically stable only below the glass transition temperature. This fact also leads to the very high thermal stability of silicones while maintaining the glass-clear optical impression.
Recently, there have been new types of highly transparent liquid silicones on the market, which inter alia can be processed by special injection-molding processes to form high-precision optical parts.
One central theme of optical applications is antireflection, since without antireflection several percent of the incident light is lost by reflection at any surface in the optical system. All silicones exhibit a certain elastic behavior and relatively high thermal expansion. Permanent bonding to rigid brittle layers, such as conventional optical interference layers made of oxides constitute, is therefore difficult to achieve. Brittle oxide layers fracture and form cracks as soon as the substrate deforms. Furthermore, low molecular weight compounds are often also released from silicone components. These make durable bonding of coatings more difficult. Furthermore, silicone surfaces are generally very nonpolar, and therefore cannot be wetted and coated without activation.
There are therefore no examples in which silicones have been coated in a vacuum with conventional interference layer systems for the purpose of antireflection.
German patent document DE 10241708 B4 (U.S. counterpart application publication US 2005/0233083) and European patent document EP 2083991 B1 (U.S. counterpart application publication US 2009/0261063) describe methods with which a nanostructure, by which the reflection of the plastic substrate is reduced, is produced on the surface of a plastic substrate by means of a plasma etching process. Such a plasma etching process, however, is not suitable for the etching of silicone surfaces.